This nonprovisional application claims priority under 35 U.S.C. 517 119(a) on Patent Application No(s). 2002-173264 filed in JAPAN on Jun. 13, 2002, which is(are) herein incorporated by reference.
The present invention relates to a phosphor screen, which displays video images and characters, and a cathodoluminescence having said phosphor screen, more precisely relates to a phosphor screen capable of preventing lights from scattering among neighbor phosphor sections.
Cathodoluminescences are widely used as television receivers, display devices of computers, etc. A cathode ray tube (CRT) is an example of the cathodoluminescences. The CRT basically comprises, an anode, an electron gun, a phosphor screen and an aluminum film on the phosphor screen. A theory of the CRT will be explained. Electrons extracted form the cathode are sharply focused by electrodes of the electron gun, and a focused electron beam irradiates the phosphor screen formed on the faceplate. The irradiated phosphor screen converts energy of the invisible electron beam into visible lights. In a color CRT, a shadow mask is provided with a proper distance away from a surface of a color phosphor screen, electron beams from three electron guns pass through holes of the shadow mask and irradiate phosphor sections, which correspond to the electron beams, so that the phosphor sections emit three color lights respectively.
The lights emitted by the phosphor screen go toward an inner part of the CRT, too. By forming an aluminum reflection film on the phosphor screen, all lights emitted by the phosphor screen go toward a viewer, so that brightness of the phosphor screen observed by the viewer can be double. By coating inner faces of a funnel and a neck tube with an electrically conductive material having proper thickness and by inputting anode voltage to the electric conductive film and the aluminum film on the phosphor screen, an inside space of the CRT, which has large capacity, has a uniform electric field. The electron beam from the electron gun travels in the uniform electric field of the CRT with constant sped, which is determined by anode potential. The electron beam moving in the inside space of the CRT at constant speed is deflected by a magnetic coil, which is installed outside of the CRT, and the deflected electron beam scans on entire phosphor screen from left to right and up to down. By scanning the electron beam, tiny spots in the phosphor screen sequentially emits cathodoluminescence lights; the viewer perceives uniformly emitting screen with after image effect of the eyes. Since the scanning electron beam is deflected to sequentially irradiate the tiny spots in the phosphor screen, the CRT must have a large vacuum space and a thick glass envelope, which can withstand the vacuum therein.
When the electron beam scanning the phosphor screen of the CRT is modulated with video signals, brightness of lighting spots on the phosphor screen synchronously vary with the video signals, so that video images are shown on the phosphor screen. Since the electron beam irradiating the phosphor screen has a highly concentrated energy, e.g., 4 KW/cm2, the number of 1020 photons/(cm2, second) are emitted from the phosphor screen, so that the phosphor screen emits lights with the brightness of 15,000 cd/m2. Images on the phosphor screen in the CRT are shown with high brightness, without rise of temperature of the phosphor screen, that is a great advantage of the CRT. The CRT capable of showing highly bright images is superior to other display devices. Since the CRTs are suitable for showing digital video images, they will be used for screen units of digital television receivers.
FIG. 1 is a partial sectional view of a conventional phosphor screen of a monochrome CRT.
A display section 10 of the CRT comprises: a glass plate 11 which acts as a faceplate; a phosphor screen 12 which is formed on the faceplate 11 and made of phosphor particles 12z; and an aluminum film 13. An electron beam 14 is irradiated from an electron gun (not shown). Lights 15 emitted from the phosphor particles 12z, which are irradiated by the electron beam 14, are scattered. Resolution of images shown on the display section 10 depends on diameter of the electron beam 14. In the CRT, the electron beams 14 from the electron gun (not shown) makes the phosphor particles 12z emit lights so as to show images on the phosphor screen. The phosphor particles 12z, to which the electron beam 14 irradiates, emit the scattered lights 15, so that a viewer feels as if the neighbor phosphor particles 12z too emit lights in spite of emitting no lights. Namely, the scattered lights 15 irradiate the phosphor particles 12z, which is irradiated by no electron beam. Therefore, contrast and sharpness of images on the phosphor screen are badly influenced.
FIG. 2 is a partial sectional view of a conventional phosphor screen of a color CRT. A display section 10a of the color CRT comprises: the faceplate 11; the phosphor screen 12 which is formed on the faceplate 11 and made of phosphor particles 12z; and the aluminum film 13, as well as the monochrome CRT. Unlike the monochrome CRT, the phosphor screen 12 includes a lot of minute phosphor sections 12d, which irradiate different colors respectively and which are arranged on the faceplate 11 with prescribed separations. A black matrix film 16 is provided between the adjacent phosphor sections 12d. Ordinary thickness of the black matrices 16 is 1 xcexcm or less and thinner than that of the phosphor screen 12; the scattered lights 15 from one phosphor section 12d go into the neighbor phosphor sections 12d. Since the black matrices 16 are coated with no phosphor particles 12z or partially coated therewith, a large space exists between the aluminum film 13 and the phosphor screen 12. The large space makes a range of scatter lights 15 long, so that the scattered lights 15 irradiate the neighbor phosphor sections 12d. Influence of the scattered lights 15 is greater in the color phosphor screen than in the monochrome phosphor screen. Therefore, the images shown on the phosphor screen 12 is whitened by increasing brightness, so that the images cannot be shown with pure colors.
In the color CRT, a plurality of layers of crystallized phosphor particles, whose diameter is about 3 xcexcm, are piled so as to increase the brightness. Since the diameter of the electron beam is about 500 xcexcm, which is much greater than that of the phosphor particles, the resolution of images on the phosphor screen is basically determined with the diameter of the electron beam irradiating the phosphor screen. Electron guns, which focus the electron beam to desired resolution on the phosphor screen, have well established, and they can show images on the phosphor screen with high resolution.
Since images on display devices are watched by human eyes, they should not irritate the eyes. Irritation of the eyes relates with quality of images and brightness thereof. To comfortably watch images on display devices for a long time, the screen brightness should be properly adjusted so as not to damage the eyes. The eyes have two kinds of light sensors, depending on light intensities: one responds on ordinary light intensities (photopic vision); and the other responds on dark light intensities (scotopic vision). Images on display devices are made with the high intensities of lights, so that the viewer observes the images with the photopic vision. The human eyes have a very wide field of vision; the viewer usually observes the images on the display device against a background of a room including furniture. If the room is made light to watch the background of the room with the photopic vision, the eyes will be comfortable to watch the images and the background, with the photopic vision, for a long time. If there is a difference in brightness between the images and the background, the two sensors in the eyes simultaneously are used for watching; the images with the photopic vision and the background with the scotopic vision. Namely, the eyes do not properly adjust the unbalanced light intensities; the unadjusted eyes are damaged with watching the images for a long time.
If the brightness of the images are much greater than that of the background, the eyes can comfortably watch the images with the photopic vision. Rooms are usually illuminated with around 1,500 lux. The brightness of furniture in illuminated rooms is around 150 cd/cm2. If the viewer watches images on the screen of the display device with around 25 cm apart therefrom, which is a distance of distinct vision, preferable screen brightness is around 170-200 cd/m2. Under that condition, the viewer can watch the images on the screen and the furniture, etc. with the photopic vision without damaging his or her eyes. On the other hand, if the furniture is in a dark room, the images are watched with the photopic vision; the background is simultaneously watched with the scotopic vision. Therefore, watching of images in dark rooms for a long time will result in damage of the eyes. Preferable screen brightness, which do not damage eyes, depends on a distance between the screen and the viewer, and it will be greater than the brightness of the distance of distinct vision if the distance between the screen and the viewer is made longer. The CRTs only hold the preferable or required screen brightness, e.g., 200 cd/m2 or more, for 10,000 hours or more.
With said high screen brightness, however, the CRTs have a serious problem. Namely, images on the screen at the distance of distinct vision exhibit considerable flicker which is fluctuation of light intensities of both large and small images on the screen. When the distance between the screen and the viewer is longer than that of distinct vision, the flicker is not visible. However, human eyes are highly sensitive to minute motion of images and variation of brightness; the viewer""s eyes detect small flicker of images and flicker of the phosphor screen without reference to images, even if the viewer cannot clearly recognize the flicker. Signals of detecting the flicker are transmitted from the eyes to brain. By unconsciously detecting the flicker for a long time, the eyes are damaged, so that eye diseases, e.g., astigmatism, or headache will be caused. Therefore, the flicker must be removed. According to experiments, the flicker of CRTs is suppressed by reducing the screen brightness. In the HDTV (High Definition TV), the brightness of the phosphor screen is made lower so as to avoid the problems of flicker. In some cases, watching a television installed in a dark room will cause eye diseases, e.g., astigmatism, amblyopia, or headache. CRTs, which is capable of showing images having required brightness without flicker, is required now.
According to the published article in Journal of Materials Chemistry and Physics (volume 73, page 144-150, 2002), it has been revealed that flicker on phosphor screens are caused with staying of secondary electrons, which are inevitably emitted from phosphor particles by irradiation of an electron beam. By coating a transparent electric conductive film of a faceplate with three layers of phosphor screens and inputting anode voltage to the transparent electric conductive film, the phosphor articles of the phosphor screens are in a strong anode field, so that the secondary electrons can be removed from the phosphor screens. By removing the secondary electrons, flicker can be disappeared from screens of CRTs which show images with required screen brightness.
In the conventional CRTs, the phosphor screen directly coats the faceplate, which is made of an electrically insulating material, e.g. glass. An anode is a carbon film formed on an inner face of a funnel and perpendicularly arranged with respect to the faceplate. Therefore, only the phosphor particles arranged at a fringe of the phosphor screen are influenced with the strong anode field; the phosphor particles in a large area of the phosphor screen are influenced with a weak anode field. When a small number of the secondary electrons electrically float around surfaces of the phosphor particles, they can be collected by the anode. By collecting the secondary electrons, movement of large electron cloud, which is a crowd of the secondary electrons, will be observed as large flicker of the screen. On the other hand, movement of small electron cloud will be observed as small flicker of images. Scale of the flicker is determined by conditions of irradiating electron beams to the phosphor screen. By increasing power of the electron beam, the scale of the flicker is made larger.
Phosphor screens, which overcome the problems of flicker, have following disadvantages: low sharpness and low contrast of images; and whitening images in color CRTs. If the brightness of the phosphor screen is high, edges or outlines of images on the phosphor screen are indistinct, further other parts of the phosphor screen, to which no electron beam irradiates, are made brighter. As a result, background brightness of the entire phosphor screen increases to unacceptable level. Contrast of images, which is ratio of light intensity of image to light intensity of background, becomes to a low level. Especially, in color CRTs, color images are whitened as a result of contamination from neighbor phosphor sections in different color. Namely, pure color images cannot be shown with high brightness. Thus, to improve contrast of images, the brightness of the phosphor screen is low. These days, CRTs, which have high resolution and low brightness, have been provided so as to show high contrast images. However, as described above, the problems of eye diseases, etc., which are caused by watching images of low brightness, have never been solved. Namely, CRTs, in which the brightness of phosphor screens is increased to required level and which are capable of showing high contrast images with pure colors, are required now.
The inventor of the present invention has studied to solve the above described problems of the conventional CRTs. As a result of the study, he found that indistinctness of outlines of images and lowering contrast are not related with a diameter of electron beam, but related with the scatter of light caused by phosphor particles in a phosphor screen. Lights are scattered in the phosphor screen without reference to brightness of the phosphor screen, but the eyes can watch images with adjusting the scatter of light. Therefore, means for preventing the scatter has not been regarded as important. When light intensities are lower than a threshold value, the eyes cannot distinguish a difference of light intensities in weak and high lights. Thus, by reducing intensities of scattered lights to proper values lower than said threshold value, the brightness of the phosphor screen can be experientially adjusted so as to show images with the highest contrast. However, this cannot solve substantively, so quality of images on the phosphor screen is inferior to that of printed images. To highly improve the quality of images on the phosphor screen, the problems of the phosphor screen should be substantively solved on the basis of optical theories.
The phosphor particles of the phosphor screen are fine particles and have optical characteristics of crystals. The crystals have lack of symmetry and have the large index of refraction, so that the phosphor particles have remarkably large index of refraction. For instance, a host crystal of typical blue and green phosphors is zinc sulfide (ZnS). Refraction index of zinc sulfide is 2.39, comparable with 2.42 of diamond. A host crystal of red phosphor is yttrium oxysulfide (Y2O2S). Although there is no available data of refraction index of Y2O2S, it is empirically known that Y2O2S also has a high index of refraction, comparable with that of ZnS. When a light irradiates a phosphor particle, about 40% of the incident light reflects on a surface of the particle; 60% of the light penetrates into the particle. In the phosphor screen of the CRT, 10 billion phosphor particles are randomly arranged. Therefore, incident lights into the phosphor screen reflect on surfaces of a large number of the particles, so that the reflected lights are well randomized the directions as scattered lights in the phosphor screen.
The inventor further found that the incident visible lights into the phosphor particle repeat internal reflection therein and gets out therefrom because the phosphor particle do not have absorption band of visible lights. The lights, which get out from the particle, repeat reflection and penetration on surfaces of neighbor phosphor particles. Therefore, a spreading distance of the lights emitting in the phosphor screen is emphasized. The lights, which have emitted in the phosphor screen, reach the eyes of the viewer after the spreading distance is emphasized, so that the phosphor screen can give a wide viewing angle of images.
The scattered lights spread horizontally and vertically in the phosphor screen. The lights horizontally spread fade images on the phosphor screen and increase background brightness. Fading images and increasing background brightness make images unclear.
An object of the present invention is to provide a phosphor screen and a cathodoluminescence, which are capable of minimizing the spread of lights in the phosphor screen, so as to restrict fading images and lowering contrast.
To achieve the objects, the present invention ha s following structures. Namely, the phosphor screen of the present invention comprises a lot of minute phosphor sections, wherein the phosphor sections are respectively enclosed by barriers, which absorb visible lights and have electric conductance, and whose height is equal to or higher than a half of thickness of the phosphor sections, and the barriers are made of a material including the particles of an inorganic compound, whose average diameter is 1-8 xcexcm, and carbon particles, whose average diameter is less than 1 xcexcm.
With this structure, the scattered lights from the phosphor section do not badly influence the neighbor, so that highly clear images can be shown on the phosphor screen. Since the electrically conductive barriers collect secondary electrons, images can be shown without flicker. Preferably, the barriers are integrated with black matrices. With this structure, influence of the scattered lights to the neighbor phosphor sections can be further restricted.
Preferably, the inorganic compound is yttrium oxysulfide, aluminum oxide, titanium dioxide or zinc sulfide. By employing said compound, a physically stable state can be maintained even in a heat process of producing the cathodoluminescence, in which temperature will rise to about 450xc2x0 C. By reusing used particles, a manufacturing cost of the cathodoluminescence can be reduced.
Preferably, the barrier material includes 0.05-20 wt % of carbon particles. By employing the barrier material, amount of gasses released from the barriers to a high vacuum space can be reduced, flicker can be removed, and sharpness and contrast of images can be improved. Note that, the phosphor sections may be made with color phosphor particles or monochrome phosphor particles.
Further, the cathodoluminescence of the present invention comprises:
a faceplate;
a phosphor screen being formed on the faceplate; and
a cathode and an anode for irradiating an electron beam, which makes phosphor particles constituting the phosphor screen emit lights,
wherein the phosphor screen comprises minute phosphor sections, the phosphor sections are respectively enclosed by barriers, which absorb visible lights and have electric conductance, and whose height is equal to or higher than a half of thickness of the phosphor sections, and the barriers are made of a material including particles of an inorganic compound, whose average diameter is 1-8 xcexcm, and carbon particles, whose average diameter is less than 1 xcexcm. With this structure, highly clear images can be shown on the phosphor screen without generating flicker. The cathodoluminescence can be used for a display device capable of showing high brightness and highly clear images.
The spreading distance of the lights in the phosphor screen change with number of layers of the phosphor particles in the screen and average mean free path. The spreading of lights is widened with an increase in the number of layers of the phosphor particles. Even if the number of layers is not changed, in the case that packing density of the phosphor particles is low, the average mean free path of the scattered lights is long. Therefore, the spread distance is made longer. In the case that the phosphor particles are fully packed, an penetration distance of electrons is quite shorter than the diameter of the phosphor particle; only the phosphor particles in a first layer, shown from an electron gun, emit lights. Other phosphor particles provided between the phosphor particles emitting lights and a faceplate do not work for illuminating the screen, but work for spreading or scattering lights. If one layer of the phosphor particles are arranged on the faceplate, no phosphor particles, which scatters lights without emitting lights, exist in the phosphor screen, so that the spread of the scattered lights in the phosphor screen can be minimized. However, in the case of forming one layer of the phosphor particles, there are gaps between adjacent phosphor particles; an electron beam often directly irradiate the faceplate via the gaps. The electron beams directly irradiating the faceplate does not work for illuminating the screen, so that the brightness of the phosphor screen is quite lowered. To maximize the brightness of the phosphor screen, the phosphor particles should be packed as no gaps are seen from the electron gun. According to the book of Cathodoluminescence (page 116, chapter 7.1.5, published by Kodansha, 1990), the spreading of scattered lights is minimized with 1.4 layers of the phosphor particles.